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Greenhouse gas impact of dual stream and single stream collection and separation of recyclables
Resources, Conservation and Recycling 69 (2012) 50–56
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Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Greenhouse gas impact of dual stream and single stream collection and separation of recyclables Garrett C. Fitzgerald, Jonathan S. Krones 1 , Nickolas J. Themelis ∗ Earth Engineering Center, Columbia University, 500 W 120th st, 926, S.W. Mudd building, New York, NY 10027, United States
a r t i c l e
i n f o
Article history: Received 6 March 2011 Received in revised form 15 August 2012 Accepted 24 August 2012 Keywords: Recycling Single stream Dual stream Energy audit Carbon footprint Municipal solid waste Material recovery facility
a b s t r a c t Over the past decade communities and municipalities have been increasingly switching their recycling systems from dual stream (DS) to single stream (SS). Accordingly, material recovery facilities (MRF) have been constructed and retrofitted in order to accommodate fully commingled input streams. This transition has been driven by a variety of factors, including a general understanding that SS tends to result in increased waste diversion rates for participating communities. This paper examines the greenhouse gas emissions, or “carbon footprint,” of recycling systems before and after the transition from DS to SS. This investigation aims to assess the environmental impact of trends in the recycling industry from a holistic perspective. In our analysis we consider several communities around the U.S. on the bases of tonnage and type of material recycled, fuel and electricity consumed in collection and separation, and avoided virgin materials consumption. By examining data from a small range of communities and MRF, we arrive at three main conclusions. First, a change from DS to SS results in approximately a 50% increase in production of recyclable commodities. Second, the net result of the DS–SS transition is approximately 710 kg CO2 -equiv. avoided per metric ton of collection. Third, the emissions associated with collection and MRF operation are small in comparison to avoided emissions from avoided consumption of virgin materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the environmental hierarchy of solid waste management options, recycling is prioritized second only to source reduction and reuse (U.S. Environmental Protection Agency, 2011b). Recycling offers environmental, social, and economic benefits stemming from landfill diversion and avoidance of virgin resource consumption (Lave et al., 1999). It has been an objective of communities and recycling collection and separation firms alike to increase both recycling rates and the value and purity of recycled material streams. These objectives are potentially in conflict, for instance with the recent trend of communities switching from dual stream (DS) to single stream (SS) collection schemes (Fickes, 2005; Ryan and Hess, 2004). In DS collection, residents source-separate their recyclables into two bins, one for paper fiber (PF) and the other for commingled plastic, metal, and glass (PMG). These two streams are collected in separate trucks or in separate compartments of the same truck. The streams are separated independent from one another. In SS collection, all permitted materials are combined in a single cart,
∗ Corresponding author. Tel.: +1 212 854 2138; fax: +1 212 854 7065. E-mail address: [email protected] (N.J. Themelis). 1 Present address: Engineering Systems Division, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States. 0921-3449/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.resconrec.2012.08.006
collected in a single truck, and separated with a single, unified process. While SS collection generally boasts elevated recycling rates and allows for expedited collection, the fully commingled material stream makes separation more difficult, demanding more sophisticated – and more energy-intensive – automated equipment (Lantz and Venters, 2002). While few dispute the resource conservation benefits of recycling, it is somewhat less obvious how recycling addresses the preeminent environmental concern of our times: anthropogenic climate change. It is of interest to communities and recycling firms alike to better understand the carbon footprint of different types of recycling schemes, both in an effort to mitigate their own contribution to greenhouse gas (GHG) emissions and to avoid pursuing environmental strategies that may result in more harm than good. In this study, we compare the GHG emissions of DS and SS recycling using a systems approach informed by life cycle assessment (LCA) methodology. The main objective is to ascertain the carbon benefit or penalty associated with the transition from DS to SS recycling, considering tonnage and type of materials collected, fuel and electricity consumed in collection and separation, and conservation of virgin resources. 1.1. Background The growth of SS separation capacity in the U.S. has been nearly constant since 1995 at an average of about 14 new MRF per year
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(Berenyi, 2008). In general, the transition from DS to SS recycling in a community is accompanied by an appreciable immediate increase in collected tonnage, although the long-term effects of the transition depend greatly on numerous characteristics of the original recycling system as well as the community (De Young, 2000; Gamba and Oskamp, 1994; Gellynck et al., 2011; Halvorsen, 2008; Hornik et al., 1995; Sidique et al., 2010; Wang et al., 1997; Werner and Makela, 1998). Thus, the increase in recycling rate has been observed to vary from 10% to 100% (Abramowitz and Timpane, 2010). Opinions about potential benefits of a transition from SS to DS vary within the material recovery community. SS collection methods are generally less costly and achieve increased recovery rates while providing greater customer convenience which frequently leads to increased community participation (R. Abramowitz, personal communication June 30, 2009). Also, SS programs commonly report fewer worker compensation claims due to the increased automation of the SS system (Kinsella and Gertman, 2007; Morawski, 2009; Snow, 2003). However, the adaptation of an existing DS MRF to SS processing requires a large initial capital investment. Most modern SS recycling programs rely on automated collection, requiring the purchase of new vehicles and the replacement of old collection bins with larger, automation-compatible bins (Dreckmann, 2004, 2008). Investments and upgrades are also necessary for the MRF in the form of additional processing equipment such as optical sorters, air knives and other separation equipment. SS systems typically result in higher residue rates accompanied by a suspected increased down-cycling due to the lower quality output streams often associated with SS plants (Berenyi, 2008; Dunn, 2003; Fickes, 2006; Kinsella, 2006; Sacia and Simmons, 2006; Schaffer, 2004). The carbon footprint of waste management is a wellinvestigated area of study. Waste management is widely regarded as a major contributor to the emission of GHGs, and the IPCC sees great opportunities for emission abatement within the waste sector (Bogner et al., 2007). Most carbon analyses of waste management take an appropriate life-cycle approach to the problem, since the environmental benefits from recycling activities, for instance, are not immediately obvious. It is only when the system boundary is broadened enough to incorporate the avoided primary material consumption that the real costs and benefits emerge. Various studies have looked at the GHG emissions from waste management in general and recycling in specific (Christensen et al., 2009; Gentil et al., 2009; U.S. Environmental Protection Agency, 2006). Prior work has also examined the problem in more detail within a life-cycle phase, e.g. paper (Merrild et al., 2009), glass (Larsen et al., 2009), plastic (Astrup et al., 2009), metal (Damgaard et al., 2009), and collection and transport (Eisted et al., 2009). Chester et al. (2008) offer a precedent for assembling data presented in the previously cited studies to assess the carbon footprint of an actual recycling collection and separation system. In this study the authors examine financial flows, energy consumption, and GHG emissions of a sample DS municipal recycling case and scenarios, including a SS scenario. The authors claim that the mode of collection (DS vs. SS) has little impact on GHG emissions intensity. They calculate DS recycling avoids 1690 kg CO2 -equiv. per ton and SS avoids 1710 kg CO2 -equiv. per ton of DS, with most of the savings due to avoided virgin material use. 1.2. Research objective and method The objective of this study is to quantify the change in GHG emissions associated with a transition from DS to SS recycling. There are a number of factors potentially contributing to the carbon footprint of recycling collection and separation. In this study the system
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boundaries are limited to the tonnage and composition of collected recyclables, operation of the collection fleet, and operation of the MRF. The structural differences between DS and SS recycling indicate that these drivers may play different roles in the carbon footprints of the two systems. For instance, the higher degree of mechanization of the SS MRF may result in the consumption of more energy per ton than in DS processing. On the other hand, the increased recycling rate associated with SS collection displaces a greater amount of virgin resources extracted and processed. This study examines the relative impact of these different emissions drivers. Field data were collected for three factors examined in this study: (a) for tonnage and composition of recyclables, material audits were conducted on three mid-sized MRFs; (b) for collection fleet operation, fuel consumption was acquired from communities that have recently transitioned from DS to SS collection; and finally; (c) energy consumption in a representative MRF was subjected to an in-depth audit before and after transition from DS to SS processing. The respective GHG footprints were calculated by applying appropriate emission intensity factors to each measured data point. For the transport and MRF operation stages, emission intensities are restricted to the “use” phase of the energy, neglecting any upstream life-cycle impacts such as emissions associated with extraction and refining of fuel. For the calculation of avoided GHG emissions from increased recycling a life-cycle assessment is necessary and was carried out using the EPA Waste Reduction Model (WARM) (U.S. Environmental Protection Agency, 2010). Results are calculated as both aggregated and per-metric ton (MT) values. The aggregated values show the total GHG emissions from each step in DS and SS collection and separation. The per-MT intensity values indicate the GHG footprint that can be attributed to a given quantity of recyclables. The difference in GHG emissions is reported as a tonnage of CO2 -equivalent emissions increased or decreased due to a transition from DS to SS.
2. Data 2.1. MRF material audit Outbound composition data were collected for DS and SS operation of three medium-sized MRFs, designated A, B, and C. MRF A was converted from DS to SS in the summer of 2008. Before conversion approximately half of the recyclables entering the MRF were from residential dual stream collection and the other half from source-separated commercial generators. After conversion to SS processing, 46% was residential single stream, 20% still dual stream (from communities that have not yet converted their collection), and the remaining 34% from source-separated commercial; however, the tonnage from commercial sources remained the same. MRF B was converted in the summer of 2007 and experienced a doubling in throughput. MRF C is unusual in that before the SS conversion in early 2008 it was almost entirely a PMG facility, while residential PF was delivered to another facility. Therefore, the increase in throughput of MRF C was due largely to the addition of a paper stream that had not been separated there previously. Quarterly outbound tonnages before and after DS–SS conversion along with the material composition are shown in Table 1. It can be seen that the conversion from DS to SS results in a significant increase in outbound tonnage as well as a change in composition of the product stream. However, the change in composition seems to be highly case-specific, as there is no clear trend across the three MRFs except for an increase in residue rates from roughly 6% to 9%. In the literature SS residue rates are reported as low as 6% and as high as 17% (Stein, 2004).
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Table 1 Detailed breakdown of outbound tonnage from MRFs for quarters before and after transition from dual stream (DS) to single stream (SS). Dashes indicate the MRF did not produce the given product. MRF A
Glass Beneficial use Clear Three mix Metal Steel cans Used beverage containers (UBC) Mixed Baled recyclables Paper High grades Mixed Old corrugated cardboard (OCC) Old newsprint (ONP) Plastic #1–#7 High density polyethylene (HDPE) Mixed other Polyethylene terephthalate (PET) Polyfilm Residue Residue Total tonnage (MT) % Increase
MRF B
MRF C
Q1 08 DS
Q1 09 SS
Q4 06 DS
Q4 08 SS
Q1 07 DS
Q1 09 SS
0% 0% 11.1%
5.6% 4.8% 1.5%
0% – 46.0%
11.4% – 8.8%
45.9% – 3.6%
12.5% – 26.3%
1.7% 0.1%
1.7% 0.2%
0.9% 0.3%
1.1% 0.6%
7.4% 2.2%
4.7% 1.2%
–
–
–
–
3.9%
0%
6.3% 1.2% 39.0% 29.8%
3.8% 7.4% 28.7% 32.9%
0.5% 0.1% 18.4% 25.5%
0% 2.9% 17.7% 44.9%
0.2% 7.5% 5.4%
1.1% 14.5% 22.2%
4.4% 0% – 0% 0.1%
0.1% 1.5% – 2.0% 0%
4.7% – – – –
3.5% – – – –
16.0% 0% 0% 0.1% –
4.9% 0.9% 2.7% 0% –
6.3%
9.8%
3.6%
9.1%
7.8%
9.0%
19,143 23.3%
23,595
14,393 84.2%
26,517
21,297 49.8%
31,906
These data indicate that a conversion from DS to SS is associated with an increase of collected recyclables on average of about 50%. 2.2. MRF energy audit To address the GHG impact of the separation and processing of collected recyclables, an audit was performed on the electricity, natural gas, diesel, and support (off-site) vehicle fuel usage at MRF A, before and after transition from DS to SS operation. These numbers are presented in absolute value and normalized on the basis of inbound tonnage in Table 2. This normalization allows us to observe the change in energy intensity of processing 1 MT of recyclables, and is based on measured values of 6037 and 7990 MT per month under DS and SS operation, respectively (Table 1). The audit shows that the monthly use of electricity increased by about 58% – i.e., from 70,000 kWh to 110,000 kWh – from DS to SS. This corresponds to an energy intensity of 11.5 kWh/MT for DS and 13.8 kWh/MT for SS; this indicates that there was a 19% increase in electricity intensity due to the extra separation equipment required for SS. Natural gas consumption, which is used on-site for heating, exhibited the opposite trend with its intensity decreasing by 39% in the transition from DS to SS. However, part of this decrease may be due to differences in the severity of the winter during which data were collected. On site-diesel fuel, which is consumed by trucks and other vehicles operating at the MRF location, exhibited a 16% overall increase in consumption from DS to SS, but taking into account the increased tonnage handled in the SS operation the diesel fuel
intensity per MT of recyclables decreased by 12%. Finally, support fleet fuel, i.e., the diesel and gasoline used by the pickup trucks and other vehicles that serve the MRF, exhibited a 24% decrease in fuel intensity per MT of throughput.
2.3. Collection fleet audit Emissions from the collection of recyclables are a function of fuel consumption, which in turn is largely dependent on the duration of a collection route. Counter-intuitively, truck-hours may not increase with increased tonnage of SS recyclables; more efficient use of the available truck volume can occur by combining the paper fiber and the PMG streams. The increased packing density means that the collection route can be limited by mass rather than volume (or truck stability, which is of concern when DS is collected in two containers on a single vehicle). The use of large, fully commingled carts, while directly contributing to the increased recycling rates, also helps to increase route efficiency by facilitating fast, automated curbside pickup (Hong and Adams, 1999; Stein, 2004). Data from the city of Madison, Wisconsin provides clear evidence of the decrease in fleet size when the transition is made from DS to SS: the fleet serving Madison decreased from 17 DS trucks to 10 SS trucks (George Dreckmann, personal communication, 12 January 2010). The net annual cost of collection in Madison, including capital charges for carts and vehicles also decreased from $3.3 million to $2 million, a decrease of 40%.
Table 2 Average monthly energy and fuel consumption and intensity values for DS and SS operation of MRF A, along with percent change. Intensity values based on 6037 and 7990 MT per month for DS and SS, respectively. Energy type
Units
Electricity Natural Gas On-site Diesel Fleet Fuel
kWh GJ L L
Absolute consumption (per month) DS 69,600.0 253.6 4903.0 1041.0
Consumption intensity (per MT processed)
SS
% change
DS
SS
% change
109,982 204.7 5680.0 1041.0
+58% −19% +16% 0%
11.530 0.042 0.810 0.170
13.760 0.026 0.710 0.130
+19% −39% −12% −24%
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Table 3 Blaine and Burnsville, MN change from DS to SS collection. Blaine
Burnsville
DS Population Driver-hours/year Tonnage collected (MT) Fuel use intensity (L/MT) Change in fuel use intensity
55,000 11,533 6444 23.7 −42%
More detailed collection data was acquired for the small cities of Blaine and Burnsville, Minnesota. Table 3 presents this data for both DS and SS operation for the two cities (Abramowitz and Timpane, 2010). The tonnage of recyclables collected increased by 89% in Blaine but only 6% in Burnsville. Nevertheless, in both cases the truck collection productivity increased substantially: a 71% increase in MT per driver-hour in Blaine and a 40% increase for Burnsville. Fuel use intensity also improved by 42% in Blaine and 29% in Burnsville. Interestingly, although both cities exhibit substantial improvements in fuel use intensity, it was achieved in different ways for the two cities. In one case this was accomplished through a large increase in recycling rate with only a small increase in driver-hours, while in the other collected tonnage remained the same but collection speed increased appreciably. 3. Analysis 3.1. Avoided emissions from avoided virgin materials Avoided GHG emissions from recycling are attributable to waste diversion and decreased consumption of virgin resources. The change in avoided emissions from the DS–SS transition is calculated using the EPA WARM (U.S. Environmental Protection Agency, 2010). Unlike the MRF and collection audits, this calculation is based on life-cycle data. WARM quantifies the GHG emissions in units of kg CO2 -equiv. for 40 types of waste for five scenarios: source reduction, recycling, landfill, combustion, and composting. This calculation is performed based on SS tonnage reported by the three MRFs in Table 1. In the DS scenario, the difference between SS and DS tonnage for each type of recycled material is assumed to be disposed either to landfills and/or waste-to-energy (WTE) plants, depending on geographic area (van Haaren et al., 2010). The avoided GHG emissions in the three sample MRFs are displayed in Table 4. A weighted average of the mass-intensity of avoided emissions yields a value of 691 additional kg CO2 -equiv. avoided per MT of recycling due to the transition from DS to SS recycling. The decision to report the avoided emissions intensity of recycling in terms of SS MTs is made to facilitate comparability, and assumes that the overall waste generation in a community
SS
DS
SS
12,730 12,295 13.7
60,000 17,690 11,240 20.9 −29%
13,359 11,966 14.8
does not vary with method of recycling – only the final destination of the waste changes. To perform this correction, the recycled tonnage gap between DS and SS is allocated to DS as conventional disposal. For example, if 5 MT of paper is recycled in DS and 8 MT in SS, then the emissions calculation for DS includes 5 MT of recycled paper and 3 MT of paper sent to landfill and/or WTE, because we assume that those additional 3 MT are diverted to recycling in SS. The alternative to this is to consider just the impacts of the tonnage of recyclables collected. This changes both the absolute and intensity calculations for DS. In fact, with this narrower scope, the avoided emissions intensity of DS increases by 32%, cutting the advantage of the SS–DS transition to just 224 kg CO2 -equiv./MT. However, this value has less meaning because of the ambiguity in the unit of the denominator. An assumption of recycling rate invariance also helps to correct for the increased residue rate associated with SS recycling. Although a higher fraction of collected SS recyclables are discarded at the MRF, a similar fraction of DS recyclables is simply discarded at the source. 3.2. GHG emissions from MRF operations GHG emissions from the energy and fuel consumption at MRF A are calculated by multiplying the energy data summarized in Table 2 by the corresponding emission factor for each fuel source or the specific electricity mix in the region of the MRF. The emission factors used are standards for each fuel (2.32 kg CO2 -equiv./L gasoline, 2.66 kg CO2 -equiv./L diesel, 56.1 kg CO2 -equiv./GJ natural gas; U.S. Environmental Protection Agency, 2011a; Gómez et al., 2006) and the electricity mix is that for the state where MRF A is located (0.54 kg CO2 -equiv./kWh; Energy Information Administration, 2002). Table 5 shows the monthly averages of GHG emissions before and after DS-SS conversion. The GHG emissions when the plant accepted only DS recyclables averaged 11.15 kg CO2 -equiv./MT of inbound recyclables. After the SS retrofit, the GHG emissions averaged 11.07 kg CO2 -equiv./MT – less than a 1% decrease in carbon intensity. Emissions from electricity consumption make up the largest component of the total GHG emissions both before and after conversion, although more so after. This result indicates that the emissions from energy consumed by
Table 4 Quarterly GHG emissions (avoided emissions displayed in parentheses) for three sample MRFs, normalized to the total tonnage processed under SS collection and operation, in units of MT of CO2 equivalent emissions. Calculated using emission factors from EPA WARM (U.S. Environmental Protection Agency, 2010). MRF A
MRF B
MRF C
DS
SS
DS
SS
DS
SS
Glass Steel Aluminum Paper Plastic Mixed Residue
(621) (700) (286) (49,738) (1421) 0 1558
(863) (795) (708) (56,681) (1364) 0 1566
(2035) (478) (641) (27,799) (824) 0 215
(1646) (578) (2387) (55,741) (1531) 0 451
(3150) (3123) (7031) (14,916) (5656) (2630) 217
(3805) (2971) (5745) (39,026) (4436) 0 537
Total Emissions intensity (MT CO2 -equiv./MT SS)
(51,209) (2.17)
(58,846) (2.49)
(31,562) (1.19)
(61,433) (2.32)
(36,288) (1.14)
(55,447) (1.74)
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Table 5 Monthly greenhouse gas emissions and emissions intensity per MT of recyclables processed for DS and SS operation at MRF A, disaggregated by energy type. Energy type
Total emissions (kg CO2 -equiv./month)
Emissions intensity (kg CO2 -equiv./MT)
% of total
DS
SS
DS
DS
SS
Electricity Natural gas On-site diesel Fleet fuel
37,584 14,228 13,057 2415
59,390 11,486 15,126 2414
6.23 2.36 2.16 0.40
7.43 1.44 1.89 0.30
55.9% 21.1% 19.4% 3.6%
67.2% 13.0% 17.1% 2.7%
Total
67,284
88,416
11.15
11.07
the larger separation equipment is nearly perfectly balanced, on a per-MT basis, by the increased throughput of SS recycling. However, MRF A continues to accept DS and commercial sourceseparated streams along with SS at the facility, so the numbers presented above do not represent accurately the marginal emissions of separating 1 MT of only SS recyclables. In fact, during the first year after conversion – the time period from which the SS data in Tables 2 and 4 was drawn – the SS processing made up only 46% of total tonnage. If it is assumed that the emission intensity from the DS operation (11.15 kg CO2 -equiv./MT) stays constant for the 54% of non-SS occurring in SS operation, a weighted average can be calculated to determine the emission intensity of SS separation. From this calculation we find that the GHG intensity of processing 1 MT of SS is 10.98 kg CO2 -equiv., i.e., 0.17 kg or 1.5% below the intensity of processing 1 MT of DS. If MRF A is representative of MRF operations nation-wide, SS processing has a slightly smaller carbon footprint than DS processing. Nevertheless, this assumption needs to be corroborated by additional MRF case studies. If we limit our analysis to the more-endogenous variables (electricity and on-site diesel), we can attempt to control for the weather-induced changes in natural gas consumption. In this instance, per-MT SS emissions are 24% above per-MT DS emissions, although actual emission intensity is still quite low: 8.39 kg CO2 equiv./MT (DS) and 10.41 kg CO2 -equiv./MT (SS). 3.3. GHG emissions from collection fleet In the Minnesota communities case study introduced in Section 2.3, fuel consumption decreased by approximately 50% when collection was changed from DS to SS (i.e., by 10 and 6.1 L of diesel per MT collected in Blaine and Burnsville, respectively). Based on this limited data and the EPA tailpipe emission factor shown above, this corresponds to an average emission of 59 and 38 kg CO2 -equiv./MT of DS and SS collection, respectively. 4. Discussion The GHG emissions from the three recycling phases considered in this study for DS and SS collection and separation are summarized in Table 6. In both DS and SS, the avoided emissions from landfill and waste-to-energy diversion and secondary materials use vastly outweigh the emissions from the collection and separation of Table 6 Summary of emissions intensity due to avoided primary consumption and landfill/WTE diversion, separation at the MRF, and collection for DS and SS recycling systems. Phase
DS
SS
SS
the recycling tonnage. The difference is a factor 20 for DS and 40 for SS. Table 6 also shows that the emission intensity of SS is noticeably lower than that of DS, driven predominantly by the avoided emissions (97%) and to a small amount by increased collection efficiency (3%). The change in the emission intensity of the MRF processing stage is insignificant. This analysis is dependent on the chosen boundary conditions and system scope. In particular, the collection and MRF processing phases underestimate the full climate forcing burden allocated to those activities because they do not include the upstream GHG effects of producing the fuels used. For example, the EcoInvent lifecycle inventory data for diesel fuel allocates 0.72 kg CO2 -equiv./L of fuel for extraction, processing, and transport of the fossil fuel before consumption (Ecoinvent Centre, 2010). This is more than 25% of the diesel emissions factor from Section 3.2, which means the emissions estimates in Table 5 may need to be increased by a quarter or more to approach a holistic, life-cycle accuracy. Nevertheless, increasing calculation precision even to this scale will not change the overall conclusions, since most of the environmental impacts of SS and DS are due to avoided use of virgin materials. The fact that SS collection and processing exhibits a GHG emissions advantage over DS is encouraging, in view of the recent trend towards SS in municipal recycling. To maximize these benefits, however, it is of value to discuss the origins of these savings. Although this study presents no data with which to assess relative importance of the disaggregated drivers, this is an appropriate area for future work. First and foremost, it is clear that the greatest GHG savings comes from an increased recycling rate. Increased diversion rates associated with SS systems are thought to be due to the following characteristics:
(a) The larger size of the bins or carts provided to facilitate the automated pickup that characterizes the SS collection scheme (Eureka Recycling, 2002; Hong and Adams, 1999; Stein, 2004); (b) SS is an easier system for citizens to understand and comply with; and (c) communities who transition to SS recycling have an opportunity to also expand the range of materials that can be deposited in the recyclables container/cart (Hong and Adams, 1999).
The second-most important contributor is the emission from the collection fleet, which as has already been mentioned is governed by fuel consumption. This, in turn, is dependent on a number of other factors, including:
Change
(kg CO2 -equiv./MT) Recycling tonnage Separation Collection
−1451.62 11.15 58.31
−2142.53 11.07 37.89
−690.91 −0.08 −20.42
Total
−1382.16
−2093.57
−711.41
(a) Speed and cost of collection (automated collection requires less time per pick-up and thus less fuel burned while idling) (Bohm et al., 2010; Fickes, 2005). (b) Frequency of collection and truck load (inversely proportional, as a higher collection frequency can enable lower collection tonnage per collection route; a higher loading on the truck reduces
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overall distance travelled but increases work load on the truck engine) (Eureka Recycling, 2002) (c) Distance between community and MRF or waste transfer station (WTS); distance between WTS and MRF; and number of truck trips required at full operating capacity. Finally, although we found that MRF operations do not substantially affect the emission intensity through the DS–SS transition, this is partially a result of continued innovations in the MRF to allow more sophisticated and cost effective separation equipment (Biddle, 1998; Duffy, 2007; Egosi and Weinberg, 1998; Touart, 1998).
5. Conclusions This study showed that SS recycling (collection and separation) provides considerable GHG emission benefits over DS recycling, i.e. an additional avoided 711 kg CO2 -equiv./MT. The major contributors to these benefits are the increased rate of recycling and the corresponding increase in production of recyclable commodities by the SS MRF – an estimated increase of about 50% over DS collection. The increased production of recyclable materials at the MRF leads to a decrease in GHG emissions because recycled commodities replace virgin supplies of a higher carbon footprint. This remains the case even considering the elevated rates of residue from 6% to 9%. The study also shows that the change to SS collection results in considerably higher tonnage per driver hour, because of fewer truck trips and higher loads per truck trip. These factors decrease the fuel used per MT of material collected from what it was in DS collection. Finally, SS MRF operations retain comparable energy efficiency to DS MRF operations, so the increase in GHG emissions from the MRF is due almost entirely to increased throughput. It should be noted that, due to the complexities associated with a community’s transition from SS to DS, it is not possible to directly relate the increased material recovery to the SS processing and collection itself. During any upgrade or transition of material collection and processing many factors can affect community participation, including increased program education and awareness, larger and more convenient collection bins, and the potential for a broader material acceptability, and general societal trends towards increased recycling (Bohm et al., 2010; Gamba and Oskamp, 1994). Any community’s decision to switch to SS from an existing DS program should consider these possible changes along with a simple technology switch and will need to address end use markets in terms of the likely increased residue rates which may exceed end use buyer’s maximum contamination limits.
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Single-stream recycling. . . a double-edged sword? Am Met Mark 2003;April:10–1. Egosi N, Weinberg A. New MRF developments. Resource Recycling 1998;5:1–4. Eisted R, Larsen AW, Christensen TH. Collection, transfer and transport of waste: accounting of greenhouse gases and global warming contribution. Waste Management & Research 2009;27:738–45. Ecoinvent Centre Ecoinvent Data v2.2. St. Gallen, Switzerland: Swiss Centre for Life Cycle Inventories; 2010. Energy Information Administration. Updated state-level greenhouse gas emission coefficients for electricity generation 1998–2000. U.S. Department of Energy; 2002 April. Eureka Recycling. A comparative analysis of applied recycling collection methods in Saint Paul. Eureka Recycling 2002;May, Presented to the Saint Paul Neighborhood Energy Consortium. Fickes M. Single stream rising. Waste Age 2005;36(October (10)):42–9. Fickes M. Pain in the glass. Waste Age 2006;37(October (10)):68–71. Gamba RJ, Oskamp S. 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Contents lists available at SciVerse ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Greenhouse gas impact of dual stream and single stream collection and separation of recyclables Garrett C. Fitzgerald, Jonathan S. Krones 1 , Nickolas J. Themelis ∗ Earth Engineering Center, Columbia University, 500 W 120th st, 926, S.W. Mudd building, New York, NY 10027, United States
a r t i c l e
i n f o
Article history: Received 6 March 2011 Received in revised form 15 August 2012 Accepted 24 August 2012 Keywords: Recycling Single stream Dual stream Energy audit Carbon footprint Municipal solid waste Material recovery facility
a b s t r a c t Over the past decade communities and municipalities have been increasingly switching their recycling systems from dual stream (DS) to single stream (SS). Accordingly, material recovery facilities (MRF) have been constructed and retrofitted in order to accommodate fully commingled input streams. This transition has been driven by a variety of factors, including a general understanding that SS tends to result in increased waste diversion rates for participating communities. This paper examines the greenhouse gas emissions, or “carbon footprint,” of recycling systems before and after the transition from DS to SS. This investigation aims to assess the environmental impact of trends in the recycling industry from a holistic perspective. In our analysis we consider several communities around the U.S. on the bases of tonnage and type of material recycled, fuel and electricity consumed in collection and separation, and avoided virgin materials consumption. By examining data from a small range of communities and MRF, we arrive at three main conclusions. First, a change from DS to SS results in approximately a 50% increase in production of recyclable commodities. Second, the net result of the DS–SS transition is approximately 710 kg CO2 -equiv. avoided per metric ton of collection. Third, the emissions associated with collection and MRF operation are small in comparison to avoided emissions from avoided consumption of virgin materials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the environmental hierarchy of solid waste management options, recycling is prioritized second only to source reduction and reuse (U.S. Environmental Protection Agency, 2011b). Recycling offers environmental, social, and economic benefits stemming from landfill diversion and avoidance of virgin resource consumption (Lave et al., 1999). It has been an objective of communities and recycling collection and separation firms alike to increase both recycling rates and the value and purity of recycled material streams. These objectives are potentially in conflict, for instance with the recent trend of communities switching from dual stream (DS) to single stream (SS) collection schemes (Fickes, 2005; Ryan and Hess, 2004). In DS collection, residents source-separate their recyclables into two bins, one for paper fiber (PF) and the other for commingled plastic, metal, and glass (PMG). These two streams are collected in separate trucks or in separate compartments of the same truck. The streams are separated independent from one another. In SS collection, all permitted materials are combined in a single cart,
∗ Corresponding author. Tel.: +1 212 854 2138; fax: +1 212 854 7065. E-mail address: [email protected] (N.J. Themelis). 1 Present address: Engineering Systems Division, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States. 0921-3449/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.resconrec.2012.08.006
collected in a single truck, and separated with a single, unified process. While SS collection generally boasts elevated recycling rates and allows for expedited collection, the fully commingled material stream makes separation more difficult, demanding more sophisticated – and more energy-intensive – automated equipment (Lantz and Venters, 2002). While few dispute the resource conservation benefits of recycling, it is somewhat less obvious how recycling addresses the preeminent environmental concern of our times: anthropogenic climate change. It is of interest to communities and recycling firms alike to better understand the carbon footprint of different types of recycling schemes, both in an effort to mitigate their own contribution to greenhouse gas (GHG) emissions and to avoid pursuing environmental strategies that may result in more harm than good. In this study, we compare the GHG emissions of DS and SS recycling using a systems approach informed by life cycle assessment (LCA) methodology. The main objective is to ascertain the carbon benefit or penalty associated with the transition from DS to SS recycling, considering tonnage and type of materials collected, fuel and electricity consumed in collection and separation, and conservation of virgin resources. 1.1. Background The growth of SS separation capacity in the U.S. has been nearly constant since 1995 at an average of about 14 new MRF per year
G.C. Fitzgerald et al. / Resources, Conservation and Recycling 69 (2012) 50–56
(Berenyi, 2008). In general, the transition from DS to SS recycling in a community is accompanied by an appreciable immediate increase in collected tonnage, although the long-term effects of the transition depend greatly on numerous characteristics of the original recycling system as well as the community (De Young, 2000; Gamba and Oskamp, 1994; Gellynck et al., 2011; Halvorsen, 2008; Hornik et al., 1995; Sidique et al., 2010; Wang et al., 1997; Werner and Makela, 1998). Thus, the increase in recycling rate has been observed to vary from 10% to 100% (Abramowitz and Timpane, 2010). Opinions about potential benefits of a transition from SS to DS vary within the material recovery community. SS collection methods are generally less costly and achieve increased recovery rates while providing greater customer convenience which frequently leads to increased community participation (R. Abramowitz, personal communication June 30, 2009). Also, SS programs commonly report fewer worker compensation claims due to the increased automation of the SS system (Kinsella and Gertman, 2007; Morawski, 2009; Snow, 2003). However, the adaptation of an existing DS MRF to SS processing requires a large initial capital investment. Most modern SS recycling programs rely on automated collection, requiring the purchase of new vehicles and the replacement of old collection bins with larger, automation-compatible bins (Dreckmann, 2004, 2008). Investments and upgrades are also necessary for the MRF in the form of additional processing equipment such as optical sorters, air knives and other separation equipment. SS systems typically result in higher residue rates accompanied by a suspected increased down-cycling due to the lower quality output streams often associated with SS plants (Berenyi, 2008; Dunn, 2003; Fickes, 2006; Kinsella, 2006; Sacia and Simmons, 2006; Schaffer, 2004). The carbon footprint of waste management is a wellinvestigated area of study. Waste management is widely regarded as a major contributor to the emission of GHGs, and the IPCC sees great opportunities for emission abatement within the waste sector (Bogner et al., 2007). Most carbon analyses of waste management take an appropriate life-cycle approach to the problem, since the environmental benefits from recycling activities, for instance, are not immediately obvious. It is only when the system boundary is broadened enough to incorporate the avoided primary material consumption that the real costs and benefits emerge. Various studies have looked at the GHG emissions from waste management in general and recycling in specific (Christensen et al., 2009; Gentil et al., 2009; U.S. Environmental Protection Agency, 2006). Prior work has also examined the problem in more detail within a life-cycle phase, e.g. paper (Merrild et al., 2009), glass (Larsen et al., 2009), plastic (Astrup et al., 2009), metal (Damgaard et al., 2009), and collection and transport (Eisted et al., 2009). Chester et al. (2008) offer a precedent for assembling data presented in the previously cited studies to assess the carbon footprint of an actual recycling collection and separation system. In this study the authors examine financial flows, energy consumption, and GHG emissions of a sample DS municipal recycling case and scenarios, including a SS scenario. The authors claim that the mode of collection (DS vs. SS) has little impact on GHG emissions intensity. They calculate DS recycling avoids 1690 kg CO2 -equiv. per ton and SS avoids 1710 kg CO2 -equiv. per ton of DS, with most of the savings due to avoided virgin material use. 1.2. Research objective and method The objective of this study is to quantify the change in GHG emissions associated with a transition from DS to SS recycling. There are a number of factors potentially contributing to the carbon footprint of recycling collection and separation. In this study the system
51
boundaries are limited to the tonnage and composition of collected recyclables, operation of the collection fleet, and operation of the MRF. The structural differences between DS and SS recycling indicate that these drivers may play different roles in the carbon footprints of the two systems. For instance, the higher degree of mechanization of the SS MRF may result in the consumption of more energy per ton than in DS processing. On the other hand, the increased recycling rate associated with SS collection displaces a greater amount of virgin resources extracted and processed. This study examines the relative impact of these different emissions drivers. Field data were collected for three factors examined in this study: (a) for tonnage and composition of recyclables, material audits were conducted on three mid-sized MRFs; (b) for collection fleet operation, fuel consumption was acquired from communities that have recently transitioned from DS to SS collection; and finally; (c) energy consumption in a representative MRF was subjected to an in-depth audit before and after transition from DS to SS processing. The respective GHG footprints were calculated by applying appropriate emission intensity factors to each measured data point. For the transport and MRF operation stages, emission intensities are restricted to the “use” phase of the energy, neglecting any upstream life-cycle impacts such as emissions associated with extraction and refining of fuel. For the calculation of avoided GHG emissions from increased recycling a life-cycle assessment is necessary and was carried out using the EPA Waste Reduction Model (WARM) (U.S. Environmental Protection Agency, 2010). Results are calculated as both aggregated and per-metric ton (MT) values. The aggregated values show the total GHG emissions from each step in DS and SS collection and separation. The per-MT intensity values indicate the GHG footprint that can be attributed to a given quantity of recyclables. The difference in GHG emissions is reported as a tonnage of CO2 -equivalent emissions increased or decreased due to a transition from DS to SS.
2. Data 2.1. MRF material audit Outbound composition data were collected for DS and SS operation of three medium-sized MRFs, designated A, B, and C. MRF A was converted from DS to SS in the summer of 2008. Before conversion approximately half of the recyclables entering the MRF were from residential dual stream collection and the other half from source-separated commercial generators. After conversion to SS processing, 46% was residential single stream, 20% still dual stream (from communities that have not yet converted their collection), and the remaining 34% from source-separated commercial; however, the tonnage from commercial sources remained the same. MRF B was converted in the summer of 2007 and experienced a doubling in throughput. MRF C is unusual in that before the SS conversion in early 2008 it was almost entirely a PMG facility, while residential PF was delivered to another facility. Therefore, the increase in throughput of MRF C was due largely to the addition of a paper stream that had not been separated there previously. Quarterly outbound tonnages before and after DS–SS conversion along with the material composition are shown in Table 1. It can be seen that the conversion from DS to SS results in a significant increase in outbound tonnage as well as a change in composition of the product stream. However, the change in composition seems to be highly case-specific, as there is no clear trend across the three MRFs except for an increase in residue rates from roughly 6% to 9%. In the literature SS residue rates are reported as low as 6% and as high as 17% (Stein, 2004).
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Table 1 Detailed breakdown of outbound tonnage from MRFs for quarters before and after transition from dual stream (DS) to single stream (SS). Dashes indicate the MRF did not produce the given product. MRF A
Glass Beneficial use Clear Three mix Metal Steel cans Used beverage containers (UBC) Mixed Baled recyclables Paper High grades Mixed Old corrugated cardboard (OCC) Old newsprint (ONP) Plastic #1–#7 High density polyethylene (HDPE) Mixed other Polyethylene terephthalate (PET) Polyfilm Residue Residue Total tonnage (MT) % Increase
MRF B
MRF C
Q1 08 DS
Q1 09 SS
Q4 06 DS
Q4 08 SS
Q1 07 DS
Q1 09 SS
0% 0% 11.1%
5.6% 4.8% 1.5%
0% – 46.0%
11.4% – 8.8%
45.9% – 3.6%
12.5% – 26.3%
1.7% 0.1%
1.7% 0.2%
0.9% 0.3%
1.1% 0.6%
7.4% 2.2%
4.7% 1.2%
–
–
–
–
3.9%
0%
6.3% 1.2% 39.0% 29.8%
3.8% 7.4% 28.7% 32.9%
0.5% 0.1% 18.4% 25.5%
0% 2.9% 17.7% 44.9%
0.2% 7.5% 5.4%
1.1% 14.5% 22.2%
4.4% 0% – 0% 0.1%
0.1% 1.5% – 2.0% 0%
4.7% – – – –
3.5% – – – –
16.0% 0% 0% 0.1% –
4.9% 0.9% 2.7% 0% –
6.3%
9.8%
3.6%
9.1%
7.8%
9.0%
19,143 23.3%
23,595
14,393 84.2%
26,517
21,297 49.8%
31,906
These data indicate that a conversion from DS to SS is associated with an increase of collected recyclables on average of about 50%. 2.2. MRF energy audit To address the GHG impact of the separation and processing of collected recyclables, an audit was performed on the electricity, natural gas, diesel, and support (off-site) vehicle fuel usage at MRF A, before and after transition from DS to SS operation. These numbers are presented in absolute value and normalized on the basis of inbound tonnage in Table 2. This normalization allows us to observe the change in energy intensity of processing 1 MT of recyclables, and is based on measured values of 6037 and 7990 MT per month under DS and SS operation, respectively (Table 1). The audit shows that the monthly use of electricity increased by about 58% – i.e., from 70,000 kWh to 110,000 kWh – from DS to SS. This corresponds to an energy intensity of 11.5 kWh/MT for DS and 13.8 kWh/MT for SS; this indicates that there was a 19% increase in electricity intensity due to the extra separation equipment required for SS. Natural gas consumption, which is used on-site for heating, exhibited the opposite trend with its intensity decreasing by 39% in the transition from DS to SS. However, part of this decrease may be due to differences in the severity of the winter during which data were collected. On site-diesel fuel, which is consumed by trucks and other vehicles operating at the MRF location, exhibited a 16% overall increase in consumption from DS to SS, but taking into account the increased tonnage handled in the SS operation the diesel fuel
intensity per MT of recyclables decreased by 12%. Finally, support fleet fuel, i.e., the diesel and gasoline used by the pickup trucks and other vehicles that serve the MRF, exhibited a 24% decrease in fuel intensity per MT of throughput.
2.3. Collection fleet audit Emissions from the collection of recyclables are a function of fuel consumption, which in turn is largely dependent on the duration of a collection route. Counter-intuitively, truck-hours may not increase with increased tonnage of SS recyclables; more efficient use of the available truck volume can occur by combining the paper fiber and the PMG streams. The increased packing density means that the collection route can be limited by mass rather than volume (or truck stability, which is of concern when DS is collected in two containers on a single vehicle). The use of large, fully commingled carts, while directly contributing to the increased recycling rates, also helps to increase route efficiency by facilitating fast, automated curbside pickup (Hong and Adams, 1999; Stein, 2004). Data from the city of Madison, Wisconsin provides clear evidence of the decrease in fleet size when the transition is made from DS to SS: the fleet serving Madison decreased from 17 DS trucks to 10 SS trucks (George Dreckmann, personal communication, 12 January 2010). The net annual cost of collection in Madison, including capital charges for carts and vehicles also decreased from $3.3 million to $2 million, a decrease of 40%.
Table 2 Average monthly energy and fuel consumption and intensity values for DS and SS operation of MRF A, along with percent change. Intensity values based on 6037 and 7990 MT per month for DS and SS, respectively. Energy type
Units
Electricity Natural Gas On-site Diesel Fleet Fuel
kWh GJ L L
Absolute consumption (per month) DS 69,600.0 253.6 4903.0 1041.0
Consumption intensity (per MT processed)
SS
% change
DS
SS
% change
109,982 204.7 5680.0 1041.0
+58% −19% +16% 0%
11.530 0.042 0.810 0.170
13.760 0.026 0.710 0.130
+19% −39% −12% −24%
G.C. Fitzgerald et al. / Resources, Conservation and Recycling 69 (2012) 50–56
53
Table 3 Blaine and Burnsville, MN change from DS to SS collection. Blaine
Burnsville
DS Population Driver-hours/year Tonnage collected (MT) Fuel use intensity (L/MT) Change in fuel use intensity
55,000 11,533 6444 23.7 −42%
More detailed collection data was acquired for the small cities of Blaine and Burnsville, Minnesota. Table 3 presents this data for both DS and SS operation for the two cities (Abramowitz and Timpane, 2010). The tonnage of recyclables collected increased by 89% in Blaine but only 6% in Burnsville. Nevertheless, in both cases the truck collection productivity increased substantially: a 71% increase in MT per driver-hour in Blaine and a 40% increase for Burnsville. Fuel use intensity also improved by 42% in Blaine and 29% in Burnsville. Interestingly, although both cities exhibit substantial improvements in fuel use intensity, it was achieved in different ways for the two cities. In one case this was accomplished through a large increase in recycling rate with only a small increase in driver-hours, while in the other collected tonnage remained the same but collection speed increased appreciably. 3. Analysis 3.1. Avoided emissions from avoided virgin materials Avoided GHG emissions from recycling are attributable to waste diversion and decreased consumption of virgin resources. The change in avoided emissions from the DS–SS transition is calculated using the EPA WARM (U.S. Environmental Protection Agency, 2010). Unlike the MRF and collection audits, this calculation is based on life-cycle data. WARM quantifies the GHG emissions in units of kg CO2 -equiv. for 40 types of waste for five scenarios: source reduction, recycling, landfill, combustion, and composting. This calculation is performed based on SS tonnage reported by the three MRFs in Table 1. In the DS scenario, the difference between SS and DS tonnage for each type of recycled material is assumed to be disposed either to landfills and/or waste-to-energy (WTE) plants, depending on geographic area (van Haaren et al., 2010). The avoided GHG emissions in the three sample MRFs are displayed in Table 4. A weighted average of the mass-intensity of avoided emissions yields a value of 691 additional kg CO2 -equiv. avoided per MT of recycling due to the transition from DS to SS recycling. The decision to report the avoided emissions intensity of recycling in terms of SS MTs is made to facilitate comparability, and assumes that the overall waste generation in a community
SS
DS
SS
12,730 12,295 13.7
60,000 17,690 11,240 20.9 −29%
13,359 11,966 14.8
does not vary with method of recycling – only the final destination of the waste changes. To perform this correction, the recycled tonnage gap between DS and SS is allocated to DS as conventional disposal. For example, if 5 MT of paper is recycled in DS and 8 MT in SS, then the emissions calculation for DS includes 5 MT of recycled paper and 3 MT of paper sent to landfill and/or WTE, because we assume that those additional 3 MT are diverted to recycling in SS. The alternative to this is to consider just the impacts of the tonnage of recyclables collected. This changes both the absolute and intensity calculations for DS. In fact, with this narrower scope, the avoided emissions intensity of DS increases by 32%, cutting the advantage of the SS–DS transition to just 224 kg CO2 -equiv./MT. However, this value has less meaning because of the ambiguity in the unit of the denominator. An assumption of recycling rate invariance also helps to correct for the increased residue rate associated with SS recycling. Although a higher fraction of collected SS recyclables are discarded at the MRF, a similar fraction of DS recyclables is simply discarded at the source. 3.2. GHG emissions from MRF operations GHG emissions from the energy and fuel consumption at MRF A are calculated by multiplying the energy data summarized in Table 2 by the corresponding emission factor for each fuel source or the specific electricity mix in the region of the MRF. The emission factors used are standards for each fuel (2.32 kg CO2 -equiv./L gasoline, 2.66 kg CO2 -equiv./L diesel, 56.1 kg CO2 -equiv./GJ natural gas; U.S. Environmental Protection Agency, 2011a; Gómez et al., 2006) and the electricity mix is that for the state where MRF A is located (0.54 kg CO2 -equiv./kWh; Energy Information Administration, 2002). Table 5 shows the monthly averages of GHG emissions before and after DS-SS conversion. The GHG emissions when the plant accepted only DS recyclables averaged 11.15 kg CO2 -equiv./MT of inbound recyclables. After the SS retrofit, the GHG emissions averaged 11.07 kg CO2 -equiv./MT – less than a 1% decrease in carbon intensity. Emissions from electricity consumption make up the largest component of the total GHG emissions both before and after conversion, although more so after. This result indicates that the emissions from energy consumed by
Table 4 Quarterly GHG emissions (avoided emissions displayed in parentheses) for three sample MRFs, normalized to the total tonnage processed under SS collection and operation, in units of MT of CO2 equivalent emissions. Calculated using emission factors from EPA WARM (U.S. Environmental Protection Agency, 2010). MRF A
MRF B
MRF C
DS
SS
DS
SS
DS
SS
Glass Steel Aluminum Paper Plastic Mixed Residue
(621) (700) (286) (49,738) (1421) 0 1558
(863) (795) (708) (56,681) (1364) 0 1566
(2035) (478) (641) (27,799) (824) 0 215
(1646) (578) (2387) (55,741) (1531) 0 451
(3150) (3123) (7031) (14,916) (5656) (2630) 217
(3805) (2971) (5745) (39,026) (4436) 0 537
Total Emissions intensity (MT CO2 -equiv./MT SS)
(51,209) (2.17)
(58,846) (2.49)
(31,562) (1.19)
(61,433) (2.32)
(36,288) (1.14)
(55,447) (1.74)
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G.C. Fitzgerald et al. / Resources, Conservation and Recycling 69 (2012) 50–56
Table 5 Monthly greenhouse gas emissions and emissions intensity per MT of recyclables processed for DS and SS operation at MRF A, disaggregated by energy type. Energy type
Total emissions (kg CO2 -equiv./month)
Emissions intensity (kg CO2 -equiv./MT)
% of total
DS
SS
DS
DS
SS
Electricity Natural gas On-site diesel Fleet fuel
37,584 14,228 13,057 2415
59,390 11,486 15,126 2414
6.23 2.36 2.16 0.40
7.43 1.44 1.89 0.30
55.9% 21.1% 19.4% 3.6%
67.2% 13.0% 17.1% 2.7%
Total
67,284
88,416
11.15
11.07
the larger separation equipment is nearly perfectly balanced, on a per-MT basis, by the increased throughput of SS recycling. However, MRF A continues to accept DS and commercial sourceseparated streams along with SS at the facility, so the numbers presented above do not represent accurately the marginal emissions of separating 1 MT of only SS recyclables. In fact, during the first year after conversion – the time period from which the SS data in Tables 2 and 4 was drawn – the SS processing made up only 46% of total tonnage. If it is assumed that the emission intensity from the DS operation (11.15 kg CO2 -equiv./MT) stays constant for the 54% of non-SS occurring in SS operation, a weighted average can be calculated to determine the emission intensity of SS separation. From this calculation we find that the GHG intensity of processing 1 MT of SS is 10.98 kg CO2 -equiv., i.e., 0.17 kg or 1.5% below the intensity of processing 1 MT of DS. If MRF A is representative of MRF operations nation-wide, SS processing has a slightly smaller carbon footprint than DS processing. Nevertheless, this assumption needs to be corroborated by additional MRF case studies. If we limit our analysis to the more-endogenous variables (electricity and on-site diesel), we can attempt to control for the weather-induced changes in natural gas consumption. In this instance, per-MT SS emissions are 24% above per-MT DS emissions, although actual emission intensity is still quite low: 8.39 kg CO2 equiv./MT (DS) and 10.41 kg CO2 -equiv./MT (SS). 3.3. GHG emissions from collection fleet In the Minnesota communities case study introduced in Section 2.3, fuel consumption decreased by approximately 50% when collection was changed from DS to SS (i.e., by 10 and 6.1 L of diesel per MT collected in Blaine and Burnsville, respectively). Based on this limited data and the EPA tailpipe emission factor shown above, this corresponds to an average emission of 59 and 38 kg CO2 -equiv./MT of DS and SS collection, respectively. 4. Discussion The GHG emissions from the three recycling phases considered in this study for DS and SS collection and separation are summarized in Table 6. In both DS and SS, the avoided emissions from landfill and waste-to-energy diversion and secondary materials use vastly outweigh the emissions from the collection and separation of Table 6 Summary of emissions intensity due to avoided primary consumption and landfill/WTE diversion, separation at the MRF, and collection for DS and SS recycling systems. Phase
DS
SS
SS
the recycling tonnage. The difference is a factor 20 for DS and 40 for SS. Table 6 also shows that the emission intensity of SS is noticeably lower than that of DS, driven predominantly by the avoided emissions (97%) and to a small amount by increased collection efficiency (3%). The change in the emission intensity of the MRF processing stage is insignificant. This analysis is dependent on the chosen boundary conditions and system scope. In particular, the collection and MRF processing phases underestimate the full climate forcing burden allocated to those activities because they do not include the upstream GHG effects of producing the fuels used. For example, the EcoInvent lifecycle inventory data for diesel fuel allocates 0.72 kg CO2 -equiv./L of fuel for extraction, processing, and transport of the fossil fuel before consumption (Ecoinvent Centre, 2010). This is more than 25% of the diesel emissions factor from Section 3.2, which means the emissions estimates in Table 5 may need to be increased by a quarter or more to approach a holistic, life-cycle accuracy. Nevertheless, increasing calculation precision even to this scale will not change the overall conclusions, since most of the environmental impacts of SS and DS are due to avoided use of virgin materials. The fact that SS collection and processing exhibits a GHG emissions advantage over DS is encouraging, in view of the recent trend towards SS in municipal recycling. To maximize these benefits, however, it is of value to discuss the origins of these savings. Although this study presents no data with which to assess relative importance of the disaggregated drivers, this is an appropriate area for future work. First and foremost, it is clear that the greatest GHG savings comes from an increased recycling rate. Increased diversion rates associated with SS systems are thought to be due to the following characteristics:
(a) The larger size of the bins or carts provided to facilitate the automated pickup that characterizes the SS collection scheme (Eureka Recycling, 2002; Hong and Adams, 1999; Stein, 2004); (b) SS is an easier system for citizens to understand and comply with; and (c) communities who transition to SS recycling have an opportunity to also expand the range of materials that can be deposited in the recyclables container/cart (Hong and Adams, 1999).
The second-most important contributor is the emission from the collection fleet, which as has already been mentioned is governed by fuel consumption. This, in turn, is dependent on a number of other factors, including:
Change
(kg CO2 -equiv./MT) Recycling tonnage Separation Collection
−1451.62 11.15 58.31
−2142.53 11.07 37.89
−690.91 −0.08 −20.42
Total
−1382.16
−2093.57
−711.41
(a) Speed and cost of collection (automated collection requires less time per pick-up and thus less fuel burned while idling) (Bohm et al., 2010; Fickes, 2005). (b) Frequency of collection and truck load (inversely proportional, as a higher collection frequency can enable lower collection tonnage per collection route; a higher loading on the truck reduces
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overall distance travelled but increases work load on the truck engine) (Eureka Recycling, 2002) (c) Distance between community and MRF or waste transfer station (WTS); distance between WTS and MRF; and number of truck trips required at full operating capacity. Finally, although we found that MRF operations do not substantially affect the emission intensity through the DS–SS transition, this is partially a result of continued innovations in the MRF to allow more sophisticated and cost effective separation equipment (Biddle, 1998; Duffy, 2007; Egosi and Weinberg, 1998; Touart, 1998).
5. Conclusions This study showed that SS recycling (collection and separation) provides considerable GHG emission benefits over DS recycling, i.e. an additional avoided 711 kg CO2 -equiv./MT. The major contributors to these benefits are the increased rate of recycling and the corresponding increase in production of recyclable commodities by the SS MRF – an estimated increase of about 50% over DS collection. The increased production of recyclable materials at the MRF leads to a decrease in GHG emissions because recycled commodities replace virgin supplies of a higher carbon footprint. This remains the case even considering the elevated rates of residue from 6% to 9%. The study also shows that the change to SS collection results in considerably higher tonnage per driver hour, because of fewer truck trips and higher loads per truck trip. These factors decrease the fuel used per MT of material collected from what it was in DS collection. Finally, SS MRF operations retain comparable energy efficiency to DS MRF operations, so the increase in GHG emissions from the MRF is due almost entirely to increased throughput. It should be noted that, due to the complexities associated with a community’s transition from SS to DS, it is not possible to directly relate the increased material recovery to the SS processing and collection itself. During any upgrade or transition of material collection and processing many factors can affect community participation, including increased program education and awareness, larger and more convenient collection bins, and the potential for a broader material acceptability, and general societal trends towards increased recycling (Bohm et al., 2010; Gamba and Oskamp, 1994). Any community’s decision to switch to SS from an existing DS program should consider these possible changes along with a simple technology switch and will need to address end use markets in terms of the likely increased residue rates which may exceed end use buyer’s maximum contamination limits.
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